1. Field of the Invention
The present invention relates generally to the fields of medicine and biology. More particularly, it concerns an improved human methionine-γ-lyase (hMGL) for use in the treatment of cancer.
2. Description of Related Art
The demand for the essential amino acid, L-methionine, is exceptionally high in cancerous tissues. Depletion of methionine has been shown to be effective in killing a wide variety of tumor types without adversely affecting non-cancerous tissues. Methionine depletion can be effected via the action of enzymes that hydrolyze the amino acid. While human methionine depleting enzymes did not previously exist, a bacterial enzyme from Pseudomonas aeruginosa, methionine-γ-lyase, was shown to be therapeutically effective in the clinic and had been evaluated in clinical trials. However, methionine-γ-lyase, being a bacterial protein, is highly immunogenic, eliciting the formation of specific antibodies, leading to adverse reactions and also reduced activity. Methionine-γ-lyase also has a very short half-life of only about 2 h in vitro and in vivo, necessitating very frequent and impractically high dosing to achieve systemic depletion.
Systemic methionine depletion is the focus of much research and has the potential to treat cancers, such as metastatic breast cancer, prostate, neuroblastoma, and pancreatic carcinoma among others. Although there is much excitement for this therapeutic approach, the bacterially-derived methionine-γ-lyase has serious shortcomings (immunogenicity and rapid deactivation in serum, as discussed above) that greatly dampen enthusiasm for its use as a chemotherapeutic agent.
Previously, an engineered human methionine-γ-lyase (hMGL-NLV) was created by introducing three key amino acid substitutions in the human enzyme cystathionine-γ-lyase (CGL): E59N, R119L, E339V. See, U.S. Pat. No. 8,709,407, which is incorporated herein by reference. Unlike native CGL, which displays essentially no catalytic activity towards L-methionine, the E59N, R119L, E339V variant enzyme (hMGL-NLV) displays robust L-methionine degrading activity in vivo and in vitro. Nonetheless, there remains a need to develop human L-methionine degrading enzymes with higher catalytic activity so that a therapeutic effect can be attained with lower dosing and/or less frequent administration of the enzyme.